9 Plant science

10 Genetics

Chemical elements and water

3.1.1 State that the most frequently occurring chemical elements in living things are carbon, hydrogen, oxygen and nitrogen.

Carbon, hydrogen, oxygen and nitrogen are the most frequently occurring chemical elements in living things.

3.1.2 State that a variety of other elements are needed by living organisms, including sulfur, calcium, phosphorus, iron and sodium.

A variety of other elements are needed by living organisms, including sulfur, calcium, phosphorus, iron and sodium.

3.1.3 State one role for each of the elements mentioned in 3.1.2.

Sulfur: Needed for the synthesis of two amino acids.

Calcium: Acts as a messenger by binding to calmodulin and a few other proteins which regulate transcription and other processes in the cell.

Phosphorus: Is part of DNA molecules and is also part of the phosphate groups in ATP.

Iron: Is needed for the synthesis of cytochromes which are proteins used during electron transport for aerobic cell respiration.

Sodium: When it enters the cytoplasm, it raises the solute concentration which causes water to enter by osmosis.

3.1.4 Draw and label a diagram showing the structure of water molecules to show their polarity and hydrogen bond formation.

3.1.5 Outline the thermal, cohesive and solvent properties of water.

Thermal properties of water include heat capacity, boiling and freezing points and the cooling effect of evaporation.

Water has a large heat capacity which means that a considerable amount of energy is needed to increase it’s temperature. This is due to the strength of the hydrogen bonds which are not easily broken. This is why the temperature of water tends to remain relatively stable. It is beneficial for aquatic animals as they use water as a habitat.

Water has a high boiling and freezing point. It boils at 100 C because the strong hydrogen bonds. All these hydrogen bonds between the water molecules need to break for the liquid to change to gas. Water becomes less dense as it gets closer to the freezing point and so ice always forms on the surface first. The high boiling point of water is vital for life on earth as if water boiled at a lower temperature the water in living organisms would start to boil and therefore these organisms would not survive.The fact that water becomes less dense as it freezes is beneficial to organisms as ice will always form at the surface of lakes or seas and by doing so it insulates the water underneath, maintaining a possible habitat for organisms to live in.

Water can evaporate at temperatures below the boiling point. Hydrogen bonds need to break for this to occur.

Cohesion is the effect of hydrogen bonds holding the water molecules together. Water moves up plants because of cohesion. Long columns of water can be sucked up from roots to leaves without the columns breaking. The hydrogen bonds keep the water molecules sticking to each other.

The solvent properties of water mean that many different substances can dissolve in it because of its polarity.

3.1.6 Explain the relationship between the properties of water and its uses in living organisms as a coolant, medium for metabolic reactions and transport medium.

Water can evaporate at temperatures below the boiling point. Hydrogen bonds need to break for this to occur. The evaporation of water cools body surfaces (sweat) and plant leaves (transpiration) by using the energy from liquid water to break the hydrogen bonds. The solvent properties of water mean that many different substances can dissolve in it because of its polarity. This allows substances to be carried in the blood and sap of plants as they dissolve in water. It also makes water a good medium for metabolic reactions.